In some paper pulping processes, a solution referred to as "oxidized white liquor" is used. Oxidized white liquor is typically made by oxidizing reducing compounds found in white liquor such as sodium sulfide, sodium polysulfide and sodium thiosulfate to form an oxidized white liquor having non-reducing compounds such as sodium sulfate therein.
A stirred tank of white liquor and either air or oxygen or a combination thereof and an external heat source is a common method of commercially producing white liquor as disclosed in U.S. Pat. Nos. 5,500,085 and 5,382,322.
The oxidation reaction of sodium sulfide is exothermic and generates a significant amount of heat. A typical stirred tank process used to oxidize sodium sulfide requires additional heat input from an external source and a long residence time in the tank for the oxidation reaction to progress to a beneficial extent. Large equipment is required to hold volumes of white liquor being oxidized. In particular two stirred tanks 10 feet in diameter and 26 feet high are used. Such large tanks require a long residence time, making them inefficient and costly.